Molecular structures of acetyl fluoride and acetyl iodide

The molecular structures of acetyl fluoride and acetyl iodide have been determined by making use of the average distances obtained in the present study together with the moments of inertia reported in the literature. The large amplitude theory for a molecule with an internal top was used in the joint analysis. The thermal-average values of internuclear distances r_g and the bond angles in the zero-point average structure Φ_z are as follows: r_g(C-O) = 1.185 ±0.002 $\text{}\text{?}$rA, r_g(C-F) = 1.362± 0.002 Å, r_g(C-C) = 1.505±0.002 Å, r_g(C-H) = 1.101 ±0.004 Å, Φ_z(OCF) = 120.7°±0.4°,Φ_z(CCF) = 110.5° ± 0.5°, Φ_z(HCH) = 109.3°±0.6° tilt(CH₃) = 0.1°±1°, for acetyl fluoride; r_g(C=O) = 1.198±0.013 $\text{}\text{?}$rA, r_g(C-I) = 2.217±0.009 Å, r_g(C-C) = 1.492±0.015 $\text{}\text{?}$rA, r_g(C-H) = 1.101 ± 0.004 Å, Φ_z(OCI) = 119.5°± 0.8°,Φ_z(CCI) = 111.7°±0.9°, Φ_z(HCH) = 110.8°±0.8° and tilt(CH₃) = 1.7°+5.4° for acetyl iodide. The uncertainties represent the estimated limits of error. The barriers V₃ to internal rotation have been reanalyzed making use of the effective moments of inertia of the methyl top estimated on the basis of the large amplitude theory and resulted in 1039 and 1176 cal mol^?1 for acetyl fluoride and acetyl iodide, respectively. The structure parameters have been compared with those of other CH₃COX (X = Cl, Br, H, CH₃) type molecules.     

Zero-point average structures of acetyl chloride and acetyl bromide

The zero-point average structures of acetyl chloride and acetyl bromide have been determined by the combined use of their moments of inertia and average distances, obtained by means of microwave spectroscopy and electron diffraction. The r_z parameters determined are as follows: r_z(CO) = 1.185 ± 0.003 Å, r_z(C-Cl) = 1.796 ± 0.002 Å, r_z(C-C) = 1.505 ± 0.003 Å, r_z(C-H) = 1.092 ± 0.005 Å, φ_z(OCCl) = 121.2 ± 0.6°, φ_z(CCCl) = 111.6 ± 0.6°, φ_z(HCH) = 108.8 ± 0.8° and tilt(CH3) = 1.3 ± 1.0°, for chloride; r_z(CO) = 1.181 ± 0.003 Å, r_z(C-Br) = 1.974 ± 0.003 Å, r_z(C-C) = 1.516 ± 0.003 Å, φ_z(OCBr) = 122.3 ± 1.5°, φ_z(CCBr) = 111.0 ± 1.5°, φ_z(HCH) = 109.9 ± 1.1°, tilt(CH₃) = 1.9 ± 1.0°, for bromide. The barriers V₃ to internal rotation have been revised to 1260 and 1256 cal mol⁻¹ for the chloride and bromide, respectively.     

Gas-phase structures of neutral silicon clusters

Vibrational spectra of neutral silicon clusters Si(n), in the size range of n = 6-10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si(9) cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si(8).     

Gas-phase structures and mass spectra of binary pentafluorides

Most known non-radioactive pentafluorides have been examined by molecular-beam mass spectrometry and by the deflection of molecular beams in inhomogeneous electric fields. Extensive association of the vapors occurs for all but the lighter pentafluorides and the interhalogens. The interhalogen pentafluorides are strongly polar, consistent with the accepted C_4v symmetry. The transition-metal and Group V pentafluorides are all non-polar, except VF₅ and CrF₅ for which temperature-dependent polarity is observed. However, uncertainty exists as to whether these observations are applicable to monomeric pentafluorides in all cases. Mass-spectral cracking patterns are presented for all species.     

Proton affinity of methyl peroxynitrate

The equilibrium structures, harmonic vibrational frequencies of methyl peroxynitrate, and structures of protonated methyl peroxynitrate have been investigated using ab initio methods. The methods include the single- and double-excitation quadratic configuration (QCISD) methods and the QCISD(T) method, which incorporates a perturbational estimate of the effects of corrected triple excitation. The lowest-energy gas-phase form of protonated methyl peroxynitrate is a complex between CH3OOH and NO2+. The CH3OOH.NO2+ complex is bound by 22 +/- 2 kcal/mol. The estimated proton affinity of methyl peroxynitrate is 178.8 +/- 3 kcal/mol. A general trend for the proton affinity of ROO-NO2 (peroxynitrates) compounds is discussed.     

Gas-phase synthesis of inorganic fullerene-like structures and inorganic nanotubes

Following the discovery of fullerenes (C₆₀) and carbon nanotubes, it was shown that nanoparticles of inorganic layered compounds, like WS₂ and MoS₂, are unstable in the planar form and they form closed cage structures with polyhedral or nanotubular shapes. Although initially the method of synthesis for the formation of such closed caged structures and nanotubes involved starting from the respective oxides, it is now well established that the gas-phase synthetic route (using metal chlorides, carbonyls etc) provides an alternative which is suitable for the synthesis of very many closed caged structures and nanotubes hitherto unknown. Various issues with this method of synthesis, including its fundamentals, mechanism, and the properties of the inorganic fullerene-like structures produced are reviewed, together with some possible applications.      

Vibronic spectra and S1 excited electronic state molecular structures for acetyl chloride and acetyl fluoride

Vapor-state absorption spectra have been recorded for acetyl fluoride and acetyl chloride and also for deuterated derivatives with path lengths up to 40 m. The origins of the S₁S₀ transitions have been derived, together with the torsional-vibration energy levels in the ground state S₀ and excited singlet state S₁. Fitting the calculated and observed rotational contours of the vibronic bands has been used to estimate the geometrical parameters in the S₁ states. The carbonyl groups in the S₁ states are nonplanar. The internal-rotation potentials have been determined for acetyl fluoride and acetyl chloride in the S₁ and S₀ states. The relative intensities of the torsional transitions in those states indicate that the minima in the potential energy are appreciably displaced along the torsional coordinate in the S₀ and S₁ states.Chemical Faculty, Lomonosov Moscow University. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 26–30, January–February, 1993.     

Gas-phase structures of aminodifluorophosphines determined using electron diffraction data and computational techniques

The molecular structures of a family of eight aminodifluorophosphines, (PF2)NRR'(R, R' = H, CH3, SiH3, GeH3, PF2), have been redetermined using gas-phase electron diffraction data and high-level ab initio molecular-orbital calculations. The SARACEN method has allowed the application of flexible restraints, giving greater accuracy and precision of structure, while the SHRINK program has allowed curvilinear corrections for vibrational effects to be applied to intramolecular distances. The more accurate structures of these eight compounds show consistent patterns of effects attributable to the various substituents, while conformations are dominated by the requirement that adjacent phosphorus and nitrogen lone pairs of electrons should be orthogonal.     

Gas-phase lanthanide chloride clusters: relationships among ESI abundances and DFT structures and energetics

Anionic lanthanide chloride clusters, Ln(n)Cl(3n+1)(-), were produced by electrospray ionization (ESI) of LnCl(3) in isopropanol, where Ln = La-Lu (except Pm); the clusters were characterized using a quadrupole ion trap mass spectrometer. High-abundance "magic number" clusters were apparent at n = 4 for the early Ln (La-Sm), and at n = 5 for the late Ln (Dy-Lu). Density functional theory computations of La(n)Cl(3n+1)(-) and Lu(n)Cl(3n+1)(-) clusters (n = 1-6) indicate that the clusters with n = 4-6 are rings with a central chlorine atom. Computed structures show six-coordinate Ln in distorted octahedral sites in "magic number" La(4)Cl(13)(-) and Lu(5)Cl(16)(-), which have particularly large dissociation energies. For lanthanum, larger anionic chloride clusters with multiple charges of down to -5 were observed; their fragmentation by collision-induced dissociation in the ion trap revealed La(4)Cl(13)(-) as a common product. Gas-phase hydrolysis to Ln(n)Cl(3n+1-y)(OH)(y)(-) (y = 1, 2) was prevalent for the late lanthanides, but only for small clusters, n = 2 or 3; larger clusters were evidently resistant to gas-phase hydrolysis. ESI of selected LnBr(3) and LnI(3) resulted in Ln(n)X(3n+1)(-) clusters (X = Br, I)--in contrast to Ln(n)Cl(3n+1)(-) clusters, the only observed (minor) high-abundance clusters were La(4)Br(13)(-) and Ce(4)Br(13)(-).     

New derivatives of trifluoroacetyl acetaldehyde and trifluoroaldol

The reaction of β-ethoxyvinyl trifluoromethyl ketone 1 with O-nucleophiles such as alcohols and diols leads to various derivatives of trifluoroacetyl acetaldehyde, such as β-alkoxyvinyl trifluoromethyl ketones 3 and cyclic keto acetals 4. Several derivatives synthesized contain chiral auxiliaries. Reduction of the compounds 1, 3, 4 under various reaction conditions leads to the trifluoroaldol derivatives 6, 7, 9, 10 containing a protected aldehyde group. The compounds obtained are useful building blocks in fluoroorganic synthesis.     

Ab initio studies of isomerization and dissociation reactions of methyl peroxynitrate

Using the CCSD(T)/cc-pVDZ//B3LYP/6-311G(2d,2p) method, we calculated the detailed potential energy surfaces (PESs) for the unimolecular isomerization and decomposition of methyl peroxynitrate (CH₃O₂NO₂). The results show that there are the two most stable isomers, IS1a and IS1b, which are a pair of mirror image isomers. From IS1a and IS1b, different isomerization and unimolecular decomposition reaction channels have been studied and discussed. Among them, the predominant thermal decomposition pathways are those leading to CH₃O₂ + NO₂ and cis-CH₃ONO + O₂. The former is the lowest-energy path through the direct O–N bond rupture in IS1a or IS1b. The PES along the O–N bond in IS1a has been scanned, where the energy of IS1a reaches maximum value of 23.5 kcal/mol when the O–N bond is stretched to about 2.8 Å. This energy is 2.7 kcal/mol larger than the O–N bond dissociation energy (BDE) and 2.8 kcal/mol larger than the experimental active energy. In addition, because the energy barriers of IS1a isomerization to IS2a are 23.8 kcal/mol, close to the 20.8 kcal/mol O–N BDE in IS1a or IS1b, the isomerization reaction may compete with the direct bond rupture dissociation reaction.     

Reaction of trifluoroacetyl isocyanate with o-sulfobenzimide

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2874–2875, December, 1990.     

Gas-phase DNA oligonucleotide structures. A QM/MM and atoms in molecules study

QM/MM calculations have been employed to investigate the role of hydrogen bonding and pi-stacking in single- and double-stranded DNA oligonucleotides. DFT calculations and Atoms in Molecules analysis on QM/MM-optimized structures allow characterization and estimation of the energies of pi-stacking and hydrogen-bond interactions. This shows that pi-stacking interactions depend on the number and the nature of the DNA bases for single-stranded nucleotides; for instance, guanines are found to be involved in strong hydrogen bonds, whereas adenines interact mainly via stacking interactions. The role of interbase hydrogen bonding was explored: the -NH2 groups of guanine, adenine, and cytosine participate in N-H...O and N-H...N interactions. These are much stronger in single-strand oligonucleotides, where the -NH2 groups are highly nonplanar. In double-stranded DNA, the strong base-pairing hydrogen bonds of complementary bases lead to more planar -NH2 groups, which tend to be involved in pi-stacking interactions rather than H-bonds. The use of AIM also allows us to evaluate the interplay of pi-stacking and H-bonding, suggesting that cooperativity does occur, but is generally limited to about 1-2 kcal/mol.     

Reaction of methylpyrimidines with trichloro- and trifluoroacetyl chlorides

The corresponding trichloro- and trifluoroacetonylpyrimidines were obtained by reaction of 2- and 4-methylpyrimidines with trichloro- and trifluoroacetyl chlorides.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1256–1258, September, 1978.     

A re-examination of the infrared and ultraviolet spectroscopy of trifluoroacetyl fluoride and trifluoroacetyl chloride: An experimental and theoretical study

The fundamental IR vibrational modes of trifluoroacetyl fluoride CF₃C(O)F and trifluoroacetyl chloride CF₃C(O)Cl have been re-examined by ab initio molecular orbital calculations and compared with literature assignments. Several bands of the IR spectrum are reassigned. The Q-branch and integrated absorption cross-sections have been measured for ν₁, ν₃, ν₄ and ν₁₁ fundamental bands for both pressurized and unpressurized samples on each molecule. The UV absorption spectra of CF₃C(O)F and CF₃C(O)Cl show a structureless continuum with a maximum at 21Onm (σ_max=3.20±0.02 × 10⁻²⁰ cm² molecule⁻¹) and 255 nm (σ_max=7.66±0.26 × 10⁻²⁰ cm² molecule⁻¹), respectively. The nature of the electronic transition giving rise to the UV absorption spectrum for CF₃C(O)F and CF₃C(O)Cl has been examined by ab initio molecular orbital calculations. It is attributed to the A¹A″←X¹A′ electronic transition.     

Reactions of trifluoroacetyl isocyanate with arylsulfamides

The reaction of trifluoroacetyl isocyanate with arylsulfamides gives N-trifluoroacetyl-N -arylsulfonylureas and 1-arylsulfonyl-1,3,5-triazine-2,4,6(1H,3H,5H)-triones in 10–15% yield.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2151–2153, September, 1990.     

Reaction of trifluoroacetyl isocyanate with Schiff bases

Conclusions Trifluoroacetyl isocyanate reacts with Schiff bases on the type of 2 + 4-cycloaddition to give substituted 4-oxo-1, 3,5-oxadiazines.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No.1, pp.202–204, January, 1976.     

Gas-phase structures of dithietane derivatives, including an electron diffraction study of 1,3-dithietane 1,1,3,3-tetraoxide

The gas electron diffraction structure of 1,3-dithietane 1,1,3,3-tetraoxide has been determined using the SARACEN method to restrain parameters that otherwise could not be refined. Quantum chemical calculations for this species showed that the potential-energy surface was extremely flat, and this was also observed from the diffraction experiments. The difference in goodness of fit for the diffraction experiment between a planar ring and one puckered by up to 9° was very small. Calculations were also performed for a variety of similar species with different numbers of oxygen atoms attached to the sulphur atoms. Topological analysis of the electron density, and electron localisation function studies of the relevant molecules, have given deeper insight into the nature of their bonding, and suggested how spatial localisation of electron pairs may influence the molecular structure.     

Gas-phase basicity of methionine

Proton affinity and protonation entropy of methionine (Met) were determined by the extended kinetic method from ESI-Q-TOF tandem mass spectrometry experiments. The values, PA(Met) = 937.5 +/- 2.9 kJ mol(-1) and Delta(p)S degrees (Met) = - 22 +/- 5 J mol(-1) K(-1), lead to gas-phase basicity GB(Met) = 898.2 +/- 3.2 kJ.mol(-1). Quantum chemical calculations using density functional theory confirm that the proton affinity of Met is indeed in the 940 kJ mol(-1) range and that a significant entropy loss, of at least - 25 J mol(-1) K(-1), occurs upon protonation. This last point is evidenced here for the first time and suggests revision of the tabulated protonation thermochemistry of Met. A comparison with previous experimental data allows us to propose the following evaluated thermochemical values: PA(Met) = 943 +/- 4 kJ mol(-1) and Delta(p)S degrees (Met) = - 35 +/- 15 J mol(-1) K(-1) and GB(Met) = 900 +/- 2 kJ mol(-1).